The term "decidual reaction" refers to the thickening and increased vascularity of the uterine lining during pregnancy. Other synonyms for this process include "decidua formation," "decidualization," and "deciduation." The decidual reaction is an essential physiological process that occurs during early pregnancy, and it helps to nourish and protect the developing fetus. The decidual reaction is characterized by the presence of specialized cells called decidual cells, which secrete hormones and other proteins that support embryonic growth. Other related terms include "trophoblast invasion" and "maternal-fetal interface," which refer to the close relationship between the maternal and fetal tissues during pregnancy.